Modems are utilized for transferring data between a personal computer (PC) and an external computer The modem translates between characters of PC data and electrical signals which are carried on the phone lines and thus enables the PC to communicate with the external world.
A modem initiates a communication with a training session in which the two communicating modems synchronize their signals and decide on a communication format. After the training session, the application prepares packets of data to transmit and provides these to the modem. The modem, in turn, converts the packets into bits or symbols, modulates the bits, samples the signal and transmits the resultant signal. After converting the phone signal to digital samples, the receiving modem demodulates the samples to determine the data of the packet. After a packet is transmitted, the two modems have a “handshake” to ensure that the receiving modem properly received the packet. The demodulated packet is then provided to the receiving application.
A modem typically is a peripheral unit of the PC, whether located externally or internally to the box of the PC. The modem includes various interface elements which digitize the analog phone signals and a digital signal processor which performs the modulation and demodulation operations, as well as the training session and any handshake operations.
Recently, a new type of modem has appeared which performs most of the modem operations internally on the personal computer. This “native” or “host” signal processing modem includes an interface which digitizes the analog phone line data and an application on the PC which performs the modem operations. Various host signal processing modems are described in U.S. Ser. No. 08/775,385 and in U.S. Pat. No. 4,965,641 which are incorporated herein to by reference.
Small to mid-sized offices which have multiple computers, some or all of whom might want to communicate with the external world, require multiple modems. Each computer can have its own individual modem or, if the computers are connected together via a local area network (LAN) 10 as shown in FIG. 1 to which reference is now made, the computers can communicate through a multi-line modem 12.
The multi-line modem 12 typically is connected to multiple external telephone lines 14 on one end and to the LAN 10 on the other. The computers of the office are also connected to the LAN 10, as “clients” 16, and there is a “server” 18 which controls the LAN 10. The multi-line modem 12 can either be an add-on card to the server 18 or a unit separate from it. The multi-line modem 12 connects the client 16 which currently wants to communicate to the line 14 currently available and performs the modem operations for each and every client currently communicating. Such multi-fine modems are also very common at web sites and Internet service providers which must provide multiple callers with access to their computers.
No multi-line modems have been produced with host signal processing because of the significant amount of processing power required to service even a single line.